Generally, there are three chemical reactors in chemical engineering, which are continuous stirred-tank reactor (CSTR), plug flow reactor (PFR) and pack-bed reactor (PBR). According to aspects of chemical engineering applications, these reactors are packed with catalysts and are often equipped with different units, such as a thermostat. Hence, a diversity of chemical reactors is designed. Key process variables for the design of a chemical reactor include flow rate, temperature, concentrations of chemical species, pressure and so on. Furthermore, the following are advantages and disadvantages of different kinds of chemical reactors.
First, the CSTR is most used in industry and can be run steadily, with reagents inside a reaction tank of the CSTR being well mixed. Additionally, several CSTRs in series use will be operated for economic benefit. However, in all kinds of continuous flowing reactors, CSTR has a lowest volumetric unit of conversion and that leads to the necessity of a reaction tank with large volume to enhance volumetric unit conversion.
The PFR, which is constructed by a single tube or many tubes in parallel, is suitable for a gas fluid. It has a high volumetric unit conversion, which is the highest in all kinds of continuously flowing reactors, and can run for long periods of time without maintenance. Disadvantages of plug flow reactors are that temperatures are hard to control due to exothermic reactions and can result in undesirable temperature gradients.
The PBR has the highest volumetric unit of conversion in all kinds of catalytic reactors. However, it is also hard to control temperature of the PBR and to replace material of a catalyst packed in the PBR.
Moreover, in order to remove organic compounds from a waste gas stream, there is an adsorbent in an absorption tower that is an example of a conventional apparatus for treating exhaust, such as active carbon. Activated carbon is a selective adsorbent, which doesn't adsorb polar material like water but does adsorb non-polar organic compounds with high molecular weights, because of its non-polarity. After adsorbing organic compounds, activated carbon can be disposed in landfill with or without incineration process, or recovered. Recovered activated carbon is regenerated by heat, vacuum desorption or stripping desorption to reuse.
Nevertheless, said conventional apparatus for treating exhaust has several drawbacks. The organic compounds are easily released from the activated carbon when environmental factors, like light or temperature, change, because the organic compounds merely physically are adsorbed by the activated carbon. Thus, saturated activated carbon should be disposed in landfill or incinerated without recovery of the organic compounds, so that using activated carbon costs much and the organic compounds can not be reused. Besides, when the activated carbon is saturated, the adsorbing processes will be stopped for replacing absorbent, such that the treatment is not continuous.
Taiwan Patent Issue No. 446572 entitled “APPARATUS AND METHOD FOR UV OXIDATION AND MICROBIOLOGICAL DECOMPOSITION OF ORGANIC WASTE AIR” is another example of a conventional apparatus for treating exhaust and discloses a first UV oxidation reactor, a humidifying device, an acid neutralizing device and a bio-filter. The first UV oxidation reactor includes an UV lamp for organic pollutants irradiation and a first monitoring device for detecting the concentration of the organic pollutants. The humidifying device and the acid neutralizer device has a specific function to humidify the organic waste air and neutralize the acidity of the organic waste air respectively. The biofilter contains biosolids with microorganisms capable of metabolizing or biodegrading one or more organic compounds and is used for treating residual pollutants after the first UV oxidation reactor. Therefore, an organic pollutant in organic waste air is photo-oxidized to reduce the concentration of the organic pollutant.
However, these past developed apparatuses for treating exhaust gas are based on the degradative oxidation reactions of organic pollutants by irradiating UV light. If some usable compounds, such as vinyl monomer, exist in the organic waste air, their degradation also occurs. Although an oxidant or a catalyst like ozone or titanium dioxide may be added in the first UV oxidation reactor, degradation of organic pollutant is enhanced without recovery the usable compounds in the organic waste air. Furthermore, amount of free radicals for decomposition of organic pollutants and generated from the catalyst of TiO2 is decreased to low efficiency of degradation of the organic pollutants, while TiO2 is exposed to light outside the first UV oxidation reactor. Hence, there is a need for an improvement over the conventional apparatus for treating exhaust.